# Explain how the structure of a membrane transport protein enables it to move specific substances across a phospholipid bilayer.

> IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks

## Mark scheme (4 marks)

1. The protein has a specific tertiary (three-dimensional) shape / conformation determined by its amino acid sequence
2. The protein contains a channel or pore / binding site that is complementary in shape (and charge) to one specific substance / substrate
3. Hydrophobic R-groups / amino acids face outward / are embedded in the hydrophobic core of the bilayer, anchoring the protein in the membrane
4. Hydrophilic R-groups line the channel / pore, allowing polar or charged substances to pass through the hydrophobic bilayer that they could not otherwise cross

## Key terms

- [phospholipid bilayer](https://www.gradenine.co.uk/glossary/phospholipid-bilayer)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-14872cb3) · Published by Druglandscape Ltd.